1. Field of the Invention
The present invention relates to an optical waveguide type biochemical sensor chip and a method of manufacturing the optical waveguide type biochemical sensor chip, and particularly to an optical waveguide type biochemical sensor chip for measuring the amount and properties of the biomolecule of organisms in an aqueous solution quantitatively and to a method of manufacturing the optical waveguide type biochemical sensor chip.
2. Description of the Related Art
Planar optical waveguide type biochemical sensor chips that are provided with a grating and a sensing membrane having a biomolecule recognition function and an information transformation function and utilize an evanescent wave arising on the surface of an optical waveguide layer have been proposed as small-sized and highly sensitive biochemical sensor chips.
In, for example, Jpn. Pat. Appln. KOKAI Publication No. 8-285851, a fluorescent immune sensor is disclosed which has a structure in which an optical waveguide layer made of a silicon oxide film having a film thickness of 620 nm is formed on a substrate by a sol-gel method and gratings are formed on both ends of the waveguide layer. There is a description in this publication that a polyimide film may be used as the optical waveguide layer. However, there is no disclosure of the details.
Also, in Jpn. Pat. Appln. KOKAI Publication No. 9-61346, a planar optical waveguide type biochemical sensor chip is disclosed which has a structure in which gratings are formed in the vicinities of both ends of a substrate and an optical waveguide layer is formed on the surface of the substrate including the gratings. In this publication, there is a description that this optical waveguide layer is preferably a film of silicon nitride, aluminum oxide, tantalum oxide manufactured by a sputtering method or a CVD method, or a glass film manufactured by an ion exchange method. An optical waveguide layer made of a similar material is disclosed in Jpn. Pat. No. 3236199.
Jpn. Pat. Appln. KOKAI Publication No. 2004-333250 discloses an optical waveguide sensor in which a sensing membrane having a biomolecule recognition function and an information transformation function is formed on a main surface of a glass substrate of 1 mm or less in thickness, wherein light beam is made to propagate within the substrate and to reflect on the substrate-sensing membrane interface.
Each of the optical waveguide layers described in the aforementioned three documents is formed in a thickness of about 1 μm depending on its material and forming method. On the other hand, as the wavelength of light beam from a light source, those ranging from the near ultraviolet region to the visible region are used in general. Therefore, the thickness of about 1 μm is a value once to four times the thickness of the wavelength of the propagated light beam.
In the optical waveguide layer having such a low thickness, the native mode number (eigenmode) determined by each refractive index of a core layer and a clad layer and by the wavelength of incident light beam, that is, the number of incident angles enabling the light beam to be impinged with optical elements for impinging light beam, for example, the grating, is a discontinuous value less than 10. It is therefore necessary to adjust the incident angle strictly corresponding to this discontinuous value.
Also, light beam propagated in a planar optical waveguide layer is generally attenuated, for example, by diffusion at the interface between the optical waveguide layer and the clad layer. Accordingly, the number of reflections is increased with a decrease in the film thickness of the optical waveguide layer, resulting in a decrease in the intensity of the light beam to be emitted. In a biochemical sensor with an optical waveguide layer having the above thickness, the number of reflections on the surface and interface (interface between the substrate and the sensing membrane) in the optical waveguide layer is increased. Therefore, the intensity of the emitting light beam attenuates and the emitting light beam tends to be affected by extraneous light beam and noise caused by, for example, the fluctuation of the system of measurement. As a result, the problem arises that a high power light source is required to obtain actual intensity of the emitted light beam, which renders it difficult to make the whole measuring system small-sized.
Also, in an optical waveguide in which light beam is propagated while totally reflecting within a glass substrate as described in the aforementioned Jpn. Pat. Appln. KOKAI Publication No. 2004-333250, the thickness of the glass substrate is set to about 0.7 mm to 1 mm in order to propagate light beam. The number of reflections on the interface between the glass substrate and the sensing membrane is decreased to a few, and detecting sensitivity is therefore lowered. If the thickness of the glass is decreased to raise the sensitivity, this reduces physical strength, bringing about handling difficulty.
In order to utilize such an optical waveguide type biosensor chip, it is desired to use a light source, such as a laser diode, that has a low power but is small and inexpensive as the light source of the light beam which is to be impinged to the chip. In order to make it possible to utilize this light source, it is necessary to make the optical waveguide layer have a proper thickness as mentioned above and also to raise the light beam impinging and emitting efficiency.
Moreover, light beam is diffused by, for example, scratches and contamination of optical elements such as a grating, leading to decreased coupling efficiency.
In the foregoing publication of Jpn. Pat. Appln. KOKAI Publication No. 8-285851, there is a description of examples in which a grating is formed by lithography on the surface of an optical waveguide layer formed of silicon oxide or polyimide by a sol gel method. However, the surface of the grating is brought into contact with the air and therefore, the efficiency is not always high, and also no device to prevent damage and contamination to the grating part is shown.
Also, in the case where the substrate is removed and processed to form a grating structure as described in the example of the aforementioned Pat. No. 3236199, a difference in refractive index between the substrate and the optical waveguide layer is decreased, resulting in reduced diffraction efficiency.